1. Field of the Invention
This invention relates generally to a method of fabrication of ferrite phase shifters and more particularly to a method of fabricating ferromagnetic toroids using mandrels for controlling the toroidal shape during a lamination process of ferrite ceramic tape used to form the toroids.
2. Description of Related Art
Ferrite phase shifters are well known and comprise devices in which the phase of an electromagnetic wave at a given frequency propagating through a transmission line can be altered. Such devices have been extensively used in radar applications for electronic beam steering and phased array applications.
The most costly item in the fabrication of toroid ferrite phase shifters is the fabrication of ferromagnetic toroids where thin walled ferrite ceramic tubes having a rectangular cross section are conventionally made by pressing a ferrite powder into a mold followed by sintering a solid body and/or diffusion-bonding solid ferrite plates together. In order to maintain the dimensional tolerances required, particularly at high frequencies used for radar applications, for example, the ferrite tube still required some type of machining. For example, a Ka band toroid typically has walls of only 0.014 in. thick, with tolerances of xc2x10.001 in. Machining of these individual ceramic tubes, particularly the relatively weak ferrite type ceramics, is inherently expensive because of the touch labor involved which results in relatively poor yields due to breakage. Furthermore, the use of commercially available ferrite materials has led to magnetic and dielectric properties that are neither well controlled nor optimized.
One known fabrication process of which the present invention is an improvement, is shown and disclosed in U.S. Pat. No. 5,876,539 entitled xe2x80x9cFabrication of Ferrite Toroidsxe2x80x9d, issued to Alex E. Bailey, et al. on Mar. 2, 1999. This patent is assigned to the assignee of the present invention and is specifically incorporated herein by reference.
The process shown and described in U.S. Pat. No. 5,876,539 uses a ceramic tape having a predetermined dielectric and magnetic properties which is formed in the three contiguous slabs of ferrite. The ferrite slabs are then laminated at relatively high temperatures and pressure. The center slab is routed to form longitudinal slots which later comprise square openings of a toroid. The base slab and the slotted slab are laminated at moderate pressures and finally the top slab is added. The final lamination step used to attach the top slab is done at a lower pressure than that used for any of the pre-lamination steps. It uses a lower pressure for the final lamination in order to avoid collapse of the internal, unsupported cavity formed between the slots of the center slab. Use of such lower pressures results in poor bonding of the cavity walls to the upper and lower slabs, thereby forming a transition layer that is lower in density and in some instances contains rows of pores along the laminated interface after densification/sintering is complete. Cracks formed at this interface and poor bonding of the top slab have resulted in poor reliability and yield. Moreover, inclusion of pores the toroid structure is disastrous in terms of magnetic properties of the toroid and may not perform the required phase shift. The result has been extremely high cost accompanied by moderate performance. Because of this, the assignee of the present invention has developed its own process for making toroidal phase shifters.
Accordingly, it is an object of the present invention to provide an improvement in the method of fabricating ferrite toroids.
It is another object of the invention to provide improved method of fabricating ferrite toroids which improves the dimensional tolerances and therefore enhances performance of ferrite phase shifters utilizing thin walled ferrite ceramic tubes.
It is a further object of the invention to provide an improved method of fabricating relatively delicate square ferrite toroids which improves yields and therefore lowers the cost of manufacturing.
The foregoing and other objects of the invention are achieved by a fabrication process which utilizes ferrite ceramic tape having an elongation characteristic of 15-25%, and the utilization of one or more rigid mandrels which are employed in the final lamination to support the rectangular cross section of the internal cavity of a respective ferrite tube, thereby permitting the highest lamination pressure to be used in the final step, and wherein the mandrel(s) are removed prior to panel densification. These modifications work together to minimize cracks and pores in the toroids and provide an added advantage of maintaining high tolerances in the internal cavity dimensions as well as the cavity-to-cavity alignment. Such process improvements lead to increased yields and lower costs. After the final tape panel is densified, the top and bottom faces are ground and polished to provide an exact dimension on the top and bottom walls. A dual ferrite toroid can be made thereafter by aligning ferrite tube cavities and joining a pair of such toroids one on top of the other.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description, while indicating the preferred method of the invention, is provided by way of illustration only, since various changes, alterations and modifications coming within the spirit and scope of the invention will become readily apparent to one skilled in the art.